This is a randomized crossover study. Patients who have been diagnosed as COPD will be recruited from outpatient department. After signing informed consent form, patients undertake an incremental exercise test and two constant load exercise at the 70% of maximum workload achieved at a previous incremental exercise test on arm ergometer in two separate days. The constant load tests will be performed with HFNC and with nasal cannula at the same inhaled oxygen fraction in random order. The hemodynamics CO, SV, EF of all subjects during exercises will be measured by a bioelectrical impedance device (Physioflow), and cerebral oxygenation status (oxygenated hemoglobin (O2Hb), deoxygenated hemoglobin (HHb), total hemoglobin (tHb) was measured by a near infrared spectrophotometer (NIRS).
